Batteries: Going viral

Date: 21-11-2013

Source: The Economist

Modified viruses help researchers boost battery performance

BUILDING a better battery has become an intense area of research. A device that could store more power in the same amount of weight as widely used lithium-ion cells could, for instance, allow smartphones to run for weeks on a single charge or an electric car to be driven non-stop for hundreds of kilometres. Among the alternatives being explored, lithium-air batteries are a favourite. But they can be tricky to make and unreliable. Now researchers have found a way to overcome some of those shortcomings with the help of genetically modified viruses.

Using viruses to make batteries is not new:

Angela Belcher and her colleagues at the Massachusetts Institute of Technology (MIT) demonstrated in 2009 that it was possible by getting modified viruses to coat themselves with the necessary materials required for the anode and cathode in a small button-sized lithium-ion cell.

Lithium-air batteries oxidise lithium at the anode and reduce oxygen at the cathode to induce a current flow. Because the oxygen comes from the air there is no need for some of the relatively heavy internal materials used in other types of battery. That promises a greatly increased energy density(the amount of power that can be stored in a given weight of battery).

In a new paper in Nature Communications the MIT team describes using modified viruses to make a cathode for a lithium-air battery. A cathode is usually harder to produce than an anode because it needs to be highly conductive. The viruses were genetically engineered to capture molecules of manganese oxide—a popular material for building lithium-air cathodes—in a solution of water. They then bind the material into an array of manganese-oxide nanowires with rough, spiky surfaces. Unlike the smooth nanowires made with conventional chemical processes, the spikes increase the surface area available for electrochemical reactions when the battery is charged and discharged. A small quantity of metal, such as palladium, is added to boost conductivity.

Making things with viruses—in this case a common bacteriophage which infects bacteria but is harmless to humans—might seem unusual. But it is similar to the biosynthesis employed in nature.

Indeed, Dr Belcher says her work was inspired by the way an abalone is genetically programmed to collect calcium from seawater in order to grow its shell. And because the process mimics a natural technique, production can be carried out at room temperature using water-based solutions, unlike conventional methods of making cathodes which are energy-intensive, and involve high temperatures and hazardous chemicals.

The researchers think they can produce a lithium-air battery with an energy density more than twice that of the best lithium-ion cells.

That would make a lot of difference to portable electronic products. A typical lithium-ion battery can store some 150 watt-hours of electricity in one kilogram of battery—itself a huge advance over the 45-80 watt-hours of a nickel-cadmium battery, let alone an old-time lead-acid battery’s 30 watt-hours.

But there is some way to go. Lithium-air cells will have drawbacks too, such as a sensitivity to high temperature which can cause their lithium-ion cousins to burst into flames. So far, the researchers have successfully tested their viral material through 50 cycles of charging and recharging, which is encouraging but well short of the hundreds or thousands of cycles expected from a commercial battery. The MIT team could be on the right road, but more work is needed before lithium-air batteries can be used to drive an electric car two or three times farther on a single charge.